# Homework Help: Electricity flow in a light sensitive theft alarm

1. May 19, 2015

I'm not learning physics in English, so terminolgy might be wrong.

1. The problem statement, all variables and given/known data

How does this circuit function? Where do the electricty flow?

2. Relevant equations
No equations

3. The attempt at a solution
Not sure what to put here?

My question is basically how it work. As far as I know, the circuit starts at the positive side of the power source, and "splits" by some part of the electrons going to the LDR1 and the other part continues going to the right, and continues branching everytime it can. But how does the light sensitivity thing work? Where do the electricity go for it to start the siren/buzzer? There is nothing near LDR1, so what activates it and what makes it stop, if there is no resistor or anything there to stop it?

Does this make sense? New to physics.

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2. May 19, 2015

### CWatters

Is this homework?

3. May 19, 2015

### BiGyElLoWhAt

Hey @FaridAbhmad, welcome to PF.

For the useful equations, what would be nice there is the necessary voltage and current equations. (The voltage across a resistor equals what?)

The attempt at the solution should be either an attempt at a time or a frequency analysis (in general) depending on whether it's ac or dc (this one happens to be dc). So setting up your voltage equations and using V=IR should be sufficient.

4. May 19, 2015

Do you mean if it is in Ohms? The resistor is that atleast, don't even know it you use something else to measure.
100k, 10k, 4.7k ohms

I'm not supposed to calculate or something. This is just to understand how a circuit works. But I don't know how this works. I need help understanding what makes the siren start, and what makes it stop? Where does the electricity go?

EDIT: yes, it's homework.

5. May 19, 2015

### BiGyElLoWhAt

Well, without going through and analyzing it, I'm not really sure, it's a fairly complex circuit. Start at the battery, and trace every path. Write the sum of the voltage equations, make sure you keep track of the parallel and series components appropriately, and if you get stuck then maybe I can help more.

6. May 19, 2015

### CWatters

As it's homework we aren't meant to give you the answer, just hints so you can solve it yourself.

I would start at the siren/buzzer. What has to happen for the siren/buzzer to sound?

7. May 19, 2015

Alright, will continue trying.
It has to be dark. When it gets darker the siren will sound.

Can I ask another question then? It is not part of the homework but is related to this.
In class when we built this circuit. We also measured Ohms to it. I found out that the darker it is the higher the resistance is. less light = high resistance. But how does this happen?

8. May 19, 2015

### BiGyElLoWhAt

The sensors could possibly break down under light, making it easier for the current to flow.

9. May 19, 2015

### CWatters

What I meant was..

For the buzzer to sound electricity has to be flowing through it so T1 must be ON.
For T1 to be ON....

Edit: It might be easier to start with the assumption that the buzzer is OFF. That implies T1 is OFF. What about T2... Go around the circuit marking it up with approximate voltages or logic levels.

Last edited: May 19, 2015
10. May 19, 2015

### CWatters

LDR1 is a Light Dependant Resistor aka Photoresistor..

http://en.wikipedia.org/wiki/Photoresistor